The invention concerns a method for operating a nuclear magnetic resonance imaging device, wherein a continuous sequence of radio-frequency (=RF) pulses is radiated onto a sample, with W being the constant phase value for all magnetization vectors, and all magnetization vectors undergo a phase progression Fn=nΨ during the n-th sequence interval, wherein P is the number of RF pulses after which the phase angle of the magnetization vectors is repeated, the total gradient moment in one or more directions in space having a value greater than 0, and consecutive pulses exhibit a constant repetition time TR, wherein, after radiation of one RF pulse, one k-space row is acquired.
Such a method is known from Zur, Y., Wood, M. L., and Neuringer, L. J., Spoiling of transverse magnetization in steady-state sequences. Magn Reson Med, 1991. 21(2): pp. 251-63.
A radio-frequency (RF) spoiled gradient echo sequence (hereinafter also referred to as FLASH) is defined by the following features (see Zur, Y., Wood, M. L., and Neuringer, L. J., Spoiling of transverse magnetization in steady-state sequences. Magn Reson Med, 1991. 21(2): pp. 251-63; Sobol, W. T. and Gauntt, D. M., On the stationary states in gradient echo imaging. Magn Reson Imaging, 1996. 6(2): pp. 384-98; Scheffler, K., A pictorial description of steady-states in rapid magnetic resonance imaging. Concepts In Magnetic Resonance, 1999. 11(5): pp. 291-304; Denolin, V., Azizieh, C., and Metens, T., New insights into the mechanisms of signal formation in RF-Spoiled gradient echo sequences. Magn Reson Med, 2005. 54(4): pp. 937-54; Leupold, J., Hennig, J., and Scheffler, K., Moment and direction of the spoiler gradient for effective artifact suppression in RF-spoiled gradient echo imaging. Magn Reson Med, 2008. 60(1): pp. 119-27):                A continuous sequence of radio frequencies is radiated onto the sample (for example, a person), wherein consecutive pulses exhibit the constant repetition time TR.        After the RF irradiation, magnetic field gradients are created, rendering the real space of interest equivalent to a positional frequency space whose inverse Fourier-transformed space is the k space of any dimension. An additional dimension that can be introduced to the acquisition is time t, whereby the k-t space is acquired.        
After each RF pulse, one row of the k-t space is acquired, wherein one row representing a set of points that lie along a straight line in any direction in the k-t space. The signal acquired after an RF signal of the k-t space is termed “k-space row.”                Each magnetization vector M(θ)=(Mx,My,Mz) undergoes a phase progression n·Ψ during the n-th sequence interval, wherein n is the pulse number and Ψ is a constant phase angle (in degrees or rad), the “spoil increment” (Sobol, W. T. and Gauntt, D. M., On the stationary states in gradient echo imaging. Magn Reson Imaging, 1996. 6(2): pp. 384-98). Between the transverse magnetization vectors Mn+(θ)=Mx,n++iMy,n+ directly after pulse no. n and Mn+1−(θ)=Mx,n+1−+iMy,n+1− directly before pulse no. n+1, the relation Mn+1−(θ)=Mn+(θ)ein ψ therefore applies.        
This phase progression is usually realized by applying a phase φn=n(n−1)Ψ/2 to the RF pulses.                during one sequence cycle of duration TR, the total gradient moment        
                              m          tot                =                              ∫                          t              ′                                                      t                ′                            +              TR                                ⁢                                    G              ⁡                              (                t                )                                      ⁢                                                  ⁢                          ⅆ              t                                                          {        1        }            
(G(t)=is the progression over time of the gradient, t′=is any selected point in time during a repetition interval) exhibits a value greater than zero in one or more directions in space (“spoiler gradient”), so that before the following RF pulse starts, magnetization in one voxel along the direction of the spoiler gradient exhibits a varying phase angle. The sum of all magnetization vectors in a single voxel prevents artifacts caused by the varying RF pulse phase angle (Leupold, J., Hennig, J., and Scheffler, K., Moment and direction of the spoiler gradient for effective artifact suppression in RF-spoiled gradient echo imaging. Magn Reson Med, 2008. 60(1): pp. 119-27).
RF-spoiled gradient echo sequences were developed because the resulting images show a pure T1 contrast if a suitable Ψ is selected (Zur, Y., Wood, M. L., and Neuringer, L. J., Spoiling of transverse magnetization in steady-state sequences. Magn Reson Med, 1991. 21(2): pp. 251-63; Scheffler, K., A pictorial description of steady-states in rapid magnetic resonance imaging. Concepts In Magnetic Resonance, 1999. 11(5): pp. 291-304).
After several sequence cycles the magnetization vector directly after the n+1-th RF pulse (Denolin, V., Azizieh, C., and Metens, T., New insights into the mechanisms of signal formation in RF-Spoiled gradient echo sequences. Magn Reson Med, 2005. 54(4): pp. 937-54; Crawley, A. P., Wood M. L., and Henkelmann, R. M., Elimination of Transverse Coherences in FLASH MRI. Magn Reson Med, 1988. 8: pp. 248-60) isMn+1+(θ)=M+n(θ+Ψ)  {2}where Ψ is the spoil increment of the RF pulse phase and θ the phase angle through which the individual magnetization vectors continue to move during TR due to the spoiler gradient. The totality of the magnetization vectors that are subject to the same angle θ then together constitute an isochromat. Equation {2} can be interpreted in such a way that after every RF pulse, the assignment of the magnetization vector of a particular isochromat to a specified profile M changes.
Moreover, the following appliesM+n+P(θ)=M+n(θ+PΨ)=M+n(θ)  {3}whenP·Ψ=K··π,  {4}applies,wherein P and K are integers. P is the period (the number of RF pulses) after which the assignment of the magnetization profile to the individual isochromats is repeated.
The function for the magnetization vector is 2π periodic in θ and the following applies:
                                          ∫                          -              π                        π                    ⁢                                                    M                n                +                            ⁡                              (                θ                )                                      ⁢                                                  ⁢                          ⅆ              θ                                      =                  const          .                                    {        5        }            
Because of the behavior according to {2} and {5}, the state that the FLASH signal enters is called the “pseudo steady state.”
According to the amplitude G (in T/m) and the duration t of the spoiler gradient, a phase difference of the magnetization
                    Δθ        =                  2          ⁢                      π            ·            γ            ·            s            ·                          ∫                                                G                  ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                                                        {        6        }            exists after the end of the gradient over a segment s (γ being the gyromagnetic moment in Hz/T).
The “spoil moment” msp of the spoiler gradient specifies which phase difference Δθ will occur along a voxel length x in the gradient direction:
                              m          sp                =                  2          ⁢                      π            ·            γ            ·            x            ·                          ∫                                                G                  ⁡                                      (                    t                    )                                                  ⁢                                  ⅆ                  t                                                                                        {        7        }            
If msp=2π, integral {5} thus runs along exactly one voxel and the signal from the voxel remains constant.
In this way, the ghost artifacts that would occur if spoil increment Ψ alone were applied, i.e. without spoiler gradient (i.e. msp=0), are suppressed. For adequate artifact suppression, however, msp=2π (or integer multiples thereof) need not necessarily apply; artifact suppression is guaranteed by a large spread of spoil moments, wherein there is no preferred direction for the spoiler gradient (Leupold, J., Hennig, J., and Scheffler, K., Moment and direction of the spoiler gradient for effective artifact suppression in RF-spoiled gradient echo imaging. Magn Reson Med, 2008. 60(1): pp. 119-27).
The object of this invention is to increase the efficiency of the gradient spoiling and thus the efficiency of artifact suppression.